1. Field of the Invention
The present invention relates to LTE Positioning Protocol (LPP), and in particular relates to a system and method for handling an error in LPP messages exchange.
2. Description of the Related Art
3GPP (3rd Generation Partnership Project) LPP (Long Term Evolution Positioning Protocol) is used in point-to-point systems between a location server (e.g. Evolved Serving Mobile Location Center (E-SMLC)) and a target device (e.g. a UE accessing E-UTRAN) in order to position the target device using position-related measurements obtained from one or more reference sources. FIG. 1 shows the LPP configuration for control plane positioning in an E-UTRAN (Evolved Universal Terrestrial Radio Access Network).
An LPP session is used between a location server and a target device in order to obtain location related measurements or a location estimate or to transfer assistance data. A single LPP session is used to support a single location request (e.g., for a single MT-LR, MO-LR or NI-LR). Multiple LPP sessions can be used between the same endpoints to support multiple different location requests. Each LPP session comprises one or more LPP transactions, with each LPP transaction performing a single operation (e.g. capability exchange, assistance data transfer, or location information transfer). In an E-UTRAN, the LPP transactions are realized as LPP procedures.
Each LPP transaction involves the exchange of one or more LPP messages between the location server and the target device. The general format of an LPP message consists of a set of common fields followed by a body. The body (which may be empty) contains information specific to a particular message type. Each message type contains information specific to one or more positioning methods and/or information common to all positioning methods. The common fields are shown in Table 1.
TABLE 1FieldRoleTransaction IDIdentify messages belonging to the same transactionTransaction EndIndicate when a transaction (e.g. one with periodicFlagresponses) has endedSequence NumberEnable detection of a duplicate LPP message at areceiverAcknowledgementEnable an acknowledgement to be requested and/orreturned for any LPP message
However, LPP is a positioning protocol for an E-UTRAN control plane. Reliable transport functionality of the LPP is not used in the user-plane solution. Therefore, LPP elementary messages (e.g. Request and Provision of Capabilities and Location Information and Assistance Data) of LPP Procedures each include a container, and an external Protocol Data Unit (EPDU), which can be used for standardization for outside 3GPP to define their own extensions to LPP messages. The Open Mobile Alliance Location (OMA LOC) group (OMA-TS-LPPe-V1—0) works on LPP extension (LPPe).
LPP procedures include a capabilities transfer procedure, assistance data transfer procedure, location information transfer procedure, error handling procedure and abort procedure. FIG. 2 is a diagram for illustrating the LPP error handling procedure. First, a sender 12 sends an LPP message to the receiver 11 in step S101. If the receiver 11 detects that the LPP message in S101 contains an error, the receiver 11 returns an Error message in step S102 to the sender 12 indicating the error or errors and discards the LPP message received in S101. Upon receiving an Error message, the sender 12 aborts ongoing procedures associated with the LPP message or restarts the aborted procedure in response to error information.
As mentioned above, during normal LPP (e.g. Capabilities Transfer, Assistance Data Transfer, Location Information Transfer) procedures, if there are many (e.g. more than one) messages in the same transaction and an error happens, the receiver aborts ongoing procedures associated with the LPP-Transaction ID indicated in the message. However, earlier successfully delivered/acknowledged messages in this transaction will be discarded.
In another situation, if an LPP/LPPe message is too large to be transferred on the user plane, a segmented message can be used. During LPP/LPPe transfer procedures, whenever a receiver detects an error in a received segment of the LPP messages from the sender, the receiver may return an LPP Error indication to the sender indicating the error. When this happens, the transfer is terminated, which means earlier delivered/acknowledged segments of the LPP/LPPe messages will be discarded. This is an undesired circumstance, especially when the receiver has successfully received (or acknowledged) a lot of sequential segments of the LPP/LPPe messages when the error happened. The termination of the transfer may cause a lot of wasted message transmissions.
Therefore, it is desirable to have a flexible way for recovering from an LPP error message in the LPP messages exchange without aborting the ongoing transfer of LPP information, so that transactions of the LPP messages may be coordinated to avoid aborting the ongoing transfer of LPP information and reduce the waste of discarding the earlier delivered/acknowledged LPP messages because of the LPP error.